1. Field of the Invention
The present invention relates to light guide units that guide light in original document reading apparatuses.
2. Description of the Related Art
Original document reading apparatuses such as copy machines, facsimiles, image scanners, and so on typically read an image of an original document by illuminating the original document, which has been placed on a platen glass. US2008/0112166 discloses mounting a plurality of LEDs (light-emitting diodes) upon a long, thin printed wiring substrate in the longitudinal direction thereof, and illuminating an original document by converging the light from the plurality of LEDs using a light guide.
However, the following problems are present in the invention disclosed in the document US2008/0112166. As illustrated in FIGS. 1 and 2 of the document US2008/0112166, a light guide is formed of a transparent plastic, has an unevenly-thick cross-sectional shape and is long in the longitudinal direction (an X axis direction, in FIGS. 1 and 2). Accordingly, with the invention disclosed in the document US2008/0112166, a high degree of warpage will occur in the light guide in a Z axis direction and a Y axis direction, even if formation conditions (mold temperature conditions, plastic extrusion conditions, and so on) are optimal. The warpage changes illumination characteristics, and causes fogging, shininess, and so on. Fogging is a phenomenon that arises when an illumination light amount is insufficient. The occurrence of fogging produces areas in an image that are darker than their surroundings. Meanwhile, shininess is a phenomenon that occurs when a location where the illumination light amount peaks is skewed from a design location.
This becomes particularly apparent when reading a facing thick original document, at areas where the distance from the platen glass to the original document is not constant. FIGS. 12A to 12D illustrate an example of fogging and shininess occurring when reading a facing thick original document such as a book or the like. As shown in FIG. 12A, when a spread thick original document P is placed upon a platen glass 140, the central area of the original document is distanced from the platen glass 140. As shown in FIG. 12B, the reading result designed to be obtained is obtained in the case where the light guide is not warped.
However, if the light guide is warped, there are cases where fogging will occur in the center of the image, as shown in FIG. 12C, or shininess will occur in the vicinity of the center of the image, as shown in FIG. 12D.
Furthermore, recently, the light amounts of LEDs are increasing due to increases in speeds, increases in image quality, and so on of original document reading apparatuses. A rise in the temperature of an LED shortens the LED's lifespan and also leads to a drop in a light-emission amount, and it is thus necessary to efficiently radiate the heat. With the invention disclosed in The document US2008/0112166, the LEDs and the light guide are provided upon a printed wiring substrate, thus making it easy for a reduction in the lifespan of the LEDs and a drop in the light-emission amounts thereof to occur.